Wide band log periodic slot rf switch



March 10, 1970 e. M. PEACE 3,

WIDE BAND LOG PERIQbIC SLOT RF swn'cu Filed Sept. 12, 1967 2 Sheets-Sheet l INVENTOR 4 GEORGE M. PEACE 3o lav/@154, rm

ATTORNEYS G. M. PEACE WIDE BAND LOG PERIODIC .SLOT RF SWITCH 2 Sheets-Sheet 2 lNV ENTOR GEORGE M. PEACE ATTORNEYS 3,500,251 WIDE BAND LOG PERIODIC SLOT RF SWITCH George M. Peace, Oak Park, Mich., assignor to Chain Lakes Research Associates, Inc., Detroit, Mich., a corporation of Michigan Filed Sept. 12, 1967, Ser. No. 667,252 Int. Cl. H01p /12, N

US. Cl. 333-7 6 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The present invention relates to a high power microwave switching device capable of operating, without any appreciable losses, over a wide frequency band, for example as high or greater than a 10 to 1 band width. Such a switching device permits to couple an input line, such as a waveguide, to either one of two output lines, each of which may be in turn coupled to either one of two further output lines and so on, such as to provide, for example, a connection between a multi-frequency microwave RF generator, such as used in high power jammers and a multiple beam antenna system whose beam position is selected by activating the switching devices. In the operation of electronic counter measure (ECM) systems in the microwave spectrum, multibeam frequency independent antenna arrays are particularly desirable either to provide directional or sector jamming. The antennas are of a frequency independent type, such as log periodic antennas, and the appropriate beam position is selected by activating a multi-throw RF switch. Mechanized switches are often used so as to provide continuous switching of the antenna array from one beam position to another, and such switches must be designed so as to be capable of handling substantial power levels, for example, from 100 to 800 watts, and so as to be capable of operating over a wide frequency band without any appreciable loss.

SUMMARY OF THE INVENTION The present invention provides a frequency independent RF SPDT switch between an input transmission line, or waveguide, and two output transmission lines or waveguides, arranged such that the coupling from the input line to either of the output lines is achieved through an array of log periodic resonant slots. The coupling is controlled by closing or opening all the slots in a particular array by means of RF switching means, preferably PIN diodes, disposed across the individual slots. The coupling action of the slots is broad band.

In the preferred structural arrangement, the input and output lines are H-guide transmission lines which present the advantage of transmitting RF energy with a power capability greater than that of more conventional 3,500,251 Patented Mar. 10, 1970 E D m waveguides while presenting the additional advantage of not having a cut-off frequency. The input H-guide is disposed and sandwiched between the output H-guides, and an array of log periodic resonant slots is utilized to couple the input H-guide to either of the two output H-guides. Switch elements, such as PIN diodes, are disposed in the slots. By turning all the switch elements of one array on or off, according to which one of the output H-guides is sought to be coupled with the input guide, while all the switch elements of the other array are turned off or on, RF power is switched to the appropriate output H-guide. Control means, such as manually or mechanically operated switches or, preferably, solid state multi-vibrators are used to turn all the diodes in one slot array to an on state while simultaneously turning all the diodes in the other slot array to an off state and vice versa.

The many objects and advantages of the present invention will become apparent to those skilled in the art when the accompanying description of the best mode contemplated to practice the invention is read in conjunction with the accompanying drawings wherein like reference numerals refer to like or equivalent parts, and in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified schematic perspective view of a wide band RF switch according to the present invention;

FIG. 2 is a schematic exploded view of the assembly of FIG. 1;

FIG. 3 is a top plan view of the input waveguide of the assembly of FIGS. 1-2 schematically showing the log periodic arrangement of the coupling slots; and

FIG. 4 is a circuit diagram of an example of the control portion of the wide band RF switch according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and more particularly to FIGS. 1 and 2 thereof, a preferred embodiment of wide band RF switch according to the present invention comprises a first waveguide input transmission line 10 disposed in close juxtaposition with the first output waveguide line 12 and a second output waveguide line 14. As oriented in the drawings, first output Waveguide line 12 is disposed above and second output waveguide line 14 is disposed below the input waveguide line 10, the three waveguides being in contact with each other with their center lines substantially in a common plane. F. J. Teisher, in The H-Guide, a waveguide for microwaves (IRE Conv. Rec., Pt. 5, 1956), has reported the development of a new type of microwave guide which he called the H-guide. The wave propagation in H-guides in the TB (transverse electric wave) mode has since been investigated by M. Cohn (Propagation in a Dielectric Loaded Parallel Plane Waveguide, IRE Transactions,

MTT, April 1959).

Each waveguide, for example, H-guide 12 in the drawings, consists of a closed, rectangular in cross section, waveguide separated in two parallelly disposed rectangular waveguide portions 16 and 18 by a slab 20 of dielectric materal. The presence of the slab 20 of dielectric permits to realize a combination of conventional 3 the parallel plates 22 and 24, while the wave propagation in the longitudinal direction has characteristics similar to surface waves (see H. M. Barlow and I. Brown, Radio Surface Waves, Oxford Press, 1962). Each H- waveguide is provided with outer edge walls, shown respectively at 30 and 32, and with an end wall 28.

Each H-guide 10, 12 or 14, has the advantage of being substantially frequency independent and of having no cutoff frequency. However, in view of the presence of sidewalls 30 and 32 required in practical applications, there results a practical lowest operating frequency dependent from the width of each H-guide.

As shown in FIG. 1, and more particularly in FIG. 2, the height of the dielectric slab 20 remains constant from one end to the other of each H-guide, while its width is longitudinally linearly decreasing from the entrance end of the guide to the end closed by wall 28, the amount of taper of the slab 20 being dependent from the frequency range at which the waveguide is capable of operating.

Input H-guide transmission line is coupled to both output H-guide transmission lines 12 and 14 by means of log periodic resonant slots placing each waveguide portion 16 of the input H-guide line 10 with the corresponding waveguide portion 16 of output H-guide 14, and the waveguide portion 18 of the input H-guide line 10 with the corresponding waveguide portion 18 of output H- guide line 12. Such log periodic slots are shown at 34 and 36 in plates 24 and 22, respectively, of input H-guide line 10, at 36a in plate 24 of output H-guide line 12 and at 34a in plate 22 of output H-guide line 14.

It is obvious that the geometry and dimensions of the slots, and the progression of the slots location are symmetrically alike when the slot array formed by slots 34 is compared to the slot array formed by slots 36, and that the geometry, progression and location of the slot array formed by slots 34 correspond to slots 34a, while the array formed by slots 36 corresponds to the array formed by slots 36a. In each slot 34 or 36 there is disposed a PIN diode 38 for the purpose of opening or closing the slot to wave propagation. As is now well known, PIN diodes are convenient microwave switching devices (D. A. Gray, How to Design PIN-Diode Control Devices, Microwaves, November 1964, p. 22) and they may be advantageously utilized in the present invention, although it is obvious that other RF switching devices could be also used.

FIG. 3, where there is shown a schematic plan view of input H-guide line 10, schematically represents some of the parameters involved in designing a microwave switch according to the present invention.

As previously explained, the plurality of slots 34 and the plurality of slots 36 are disposed in separate arrays in the symmetrical log periodic geometry shown in the drawing. In the example of FIG. 3, the length of slots 36, or of slots 34, which are ten in number in each slot array in the example chosen, varies from length L1 to length L10 forming a geometric progression with a common ratio r, such that:

The length of the each individual slot is substantially equal to one-half the wavelength for resonant coupling, such that if it is desired to provide a switch capable of operating in a frequency range of about 2 to 10 kmc, or wavelength of to 1.5 cm. choosing a common ratio r=.75, and assuming slot 36 of length L9 being of a length equal to one-half the wavelength of the lowest frequency, or 7.5 cm.:

below.

TAB LE I Slot order Slot length (cm) )r (em The diverse distances D of the slots 36 or 38 of diverse lengths, as measured from the backwall 28 of the H-guide, are also in a log periodic progression having such common ratio r, such that:

With r=.75, and assuming D10=100 cm., the log periodic progression of the distance separating backwall 28 from a slot 36 or 38 of an appropriate order is as shown in Table II.

The slots are disposed substantially transversely to the direction of propagation of the waves through the H- guides 10, are parallel to each other and their left and right edges as seen in the drawing are disposed respectively along converging lines 40 and 42, with respect to slots 36 and between converging straight line 44 and 46, with respect to slots .36. Lines 40, 42, 44 and 46 all converge to a common point 48 situated on the backwall 28 of H-guide 10 and disposed along the center line 50 thereof and situated at the apex of the tapered slab 20 of dielectric. Each bisectrix of the angles formed respectively by lines 40 and 42 and lines 44 and 46 forms with the center line 50 of the H-guide 10 an angle comprised between about 10 and about 15. The maximum width 2a of the slab 20 of dielectric is dependent from the dielectric constant of the slab material and from the smallest wavelength propagated through the H-guide for a given power factor. Such ratio, for a dielectric constant of 2.5, is expressed by 2a/ which is generally a number comprised between .05 and 10, according to the power factors desired. For example, for a wavelength A =1.5 centimeter and a ratio of 2a/ ,=.05, the maximum width 2a of the slab of dielectric is .075 centimeter.

The total width of the H-guide is generally of the order of 10 or more times the maximum width of the slab 20 of dielectric.

As previously mentioned, in order to switch the input waves propagated within the input H-guide 10, FIGS. 1 and 2, to either one of the output H-guides 12 or 14 all the PIN diodes 38 of one array of log periodic slots 36 or 34 are turned on so as to open the slots of one array to wave propagation while all the slots of the other array are closed by shorting all the PIN diodes 38 of such other array. In other words, the PIN diodes in one periodic array are either ON or OFF; when the diodes in one array are in the ON state the diodes of the other array are in the OFF state. Such turning on and off of the PIN diodes is effected by an appropriate control means, as shown for example in the circuit diagram of FIG. 4, adapted to provide the appropriate voltage level to the control element of the PIN diodes.

PIN diodes normally require a turn-on current of 70 to 100 ma. and a back bias (turn ofi) voltage of 10 volts. Consequently for a bank of 10 diodes the turn-on current is from 700 to a 1000 ma. Although such current can be easily turned on and off by means of ordinary contact switches, manually or power operated in sequences, the circuit of FIG. 4 is preferable in order to avoid switching high currents at the contact switches. The circuit of FIG. 4 has two identical branches generally shown at 52 and 54 adapted to either turn on or off respectively, for example, all the PIN diodes in slots 34 or all the PIN diodes in slot 36. Each circuit branch, for example branch 52, includes a transistor 56, preferably a high current, high PNP transistor such as for example a 2N3054. The collector of transistor 56 is connected to a l0 volt DC source 58 through a resistor 60 and the emitter is connected to a +20 volt DC source 62. The base of transistor 56 is biased by way of resistor 64 connected between such base and the +20 volt DC source 62, resistor 64 preventing any reverse collector 'base current from turning the transistor on. The base of the transistor 56 is furthermore connected through a limiting resistor 66 and a diode 68 to a terminal 69 of a switch 70. Terminal 69 of switch 70 may be controlably connected to grounded switch terminal 72 or to terminal 74 connected in turn to a +20 volt source 76.

The collector of transistor 56 is connected to the PIN diodes through current limiting resistor 78 and a low pass filter formed by series impedance 80' and shunt capacitors 82 and 84.

Switch 70 of circuit branch 52 is mechanically interconnected with switch 86 of circuit branch 54 such that when terminal 69 of switch 70 is grounded, as shown in the drawing, corresponding terminal 85 of switch 86 is placed at the +20 voltage, and vice versa.

Referring now again to branch 52 of the circuit diagram of FIG. 4, it can be seen that when switch 70 is in the position indicated, resulting in terminal 69 being grounded, transistor 56 is non-conductive as there is no base current. Under this condition, the PIN diodes in slot 34 are all back-biased to volt through resistor 78 and 60. If switch 70 is actuated such that terminal 69 is connected to terminal 74 at a volt potential, transistor 56 is forced into saturation. Under this condition, the PIN diodes in slot 34 are switched on by the switch current value as being determined by the value of resistor 78. The value of resistor 66 is chosen to produce enough base current to insure complete saturation of transistor 56, and diode 68 insure that current will flow through resistor 66 only when switch 70 is actuated to place its terminal 69 in connection with terminal 74.

It can thus be seen that by the arrangement of FIG. 4, all the PIN diodes in one slot array are turned on while all the PIN diodes in the other slot array are turned off, and vice versa.

It is obvious that switches 78 and 60 may be power actuated, or any appropriate device such as, for example, a multi-vibrator may be used to replace the manual or motorized switches when switching times of the order of the micro-seconds are desired. A multi-vi brator 90 has two outputs connected respectively to terminal 92 and terminal 94. When one output is high the other output is low until the low output is switched to high, which occurs simultaneously with the high output being switched to low. Under those conditions, if terminals 92 and 94 are connected respectively to the base of the transistors 56 of branch 52 and branch 54, such as, by means of connections 96 and 98 shown in dotted lines, the operation of the control circuit of FIG. 4 under the action of multivibrator 90 is the full equivalent of the manual or mechanical control device under the action of switches and 86.

Although the invention has been herein shown and described with respect to a particular preferred embodiment thereof, it is obvious that departures may be made from the illustrative example herein without departing from the spirit and scope of the appended claims.

I claim:

1. A wide band RF switch for coupling a first frequency independent waveguide to one or the other of a second and third frequency independent waveguides, said switch comprising:

said first waveguide disposed between said second and third waveguides in juxtaposition therewith and the axes of said three waveguides being substantially parallel to each other;

a plurality of log periodic transverse slots forming a first slot array coupling said first waveguide to said second waveguide;

a plurality of log periodic transverse slots forming a second slot array coupling said first waveguide to said third waveguide;

a first group of RF switches each in each slot of said first slot array;

a second group of RF switches each in each slot of said second slot array; and

control means for sequentially turning said first group of RF switches on and off while simultaneously turning said second group of RF switches ofi and on.

2. The switch of claim 1 wherein said waveguides are H-guides each having along the centerline thereof a longitudinally disposed slab of dielectric of constant thickness and of linearly decreasing width.

3. The switch of claim 1 wherein each of said RF switches in a PIN diode.

4. The switch of claim 1 wherein said control means includes a multi-vibrator.

5. The switch of claim 2 wherein said first waveguide has its slab of dielectric linearly decreasing in Width in the direction of wave propagation and said second and third waveguides have each its slab of dielectric linearly decreasing in width in an opposite direction.

6. The switch of claim 5 wherein said first slot array is disposed in a common wall of said first and second waveguides on one side of the centerlines thereof and said second slot array is disposed in a common wall of said first and third waveguides on the other side of the centerline thereof.

References Cited UNITED STATES PATENTS 3,287,665 11/1966 Brunton 333--7 3,218,644 11/1965 Berry 343792.5 2,632,809 3/1953 Riblet 33310 ELI LIEBERMAN, Primary Examiner F. P. BUTLER, Assistant Examiner US. Cl. X.R. 

